Field of the Invention
This disclosure is related to the field of polymer interlayers for multiple layer panels and multiple layer panels having at least one polymer interlayer sheet. Specifically, this disclosure is related to the field of polymer interlayers having improved sound insulation properties, and more specifically, to polymer interlayers having improved sound insulation properties for use in vehicle and building glazings.
Description of Related Art
Occupants of buildings, especially in areas near airports, railways, and human activities, or where street and highway traffic noise is a problem, are increasingly interested in acoustic insulating glazing.
Automobile designers are also aware that a barrier to designing quieter car interiors is the acoustic limitations of automotive glass. As automobile manufacturers have increased their efforts to design and build quieter cars and reduce vehicle interior noise through the treatment of passenger compartment boundaries with sound-attenuating packages, automotive glazing has become the primary transmission path of wind noise, external airborne noise and structure-borne noise, and it is a major contributor to the consumer's perception of vehicle interior noise level.
Laminated glass, made of a poly(vinyl butyral) (PVB) plastic interlayer sandwiched by two panes of glass sheet, has long served for safety purposes and is commonly utilized in architectural window applications and in the windows of motor vehicles and airplanes. The main function of the interlayer in the laminated safety glass is to absorb energy resulting from impact or force applied to the glass, to keep the layers of glass bonded even when the force is applied and the glass is broken, and to prevent the glass from breaking up into sharp pieces. Less known is the advantage of laminated glass for noise attenuation. Over the past decades, architectural use of PVB laminated glass in buildings near airports and railways has served to reduce the noise levels inside the buildings, making it more comfortable for the occupants. Likewise this technology is now being used in buildings where street and highway traffic noise is a problem. Recently, advances in interlayer technology have made improved laminated glass that provides noise, vibration, and harshness improvements for automotive glass.
The sound insulation property of a glass panel can be characterized by Sound Transmission Loss (STL). It is well known that sound transmission through glass exhibits coincident effect. Glass has a specific critical or coincident frequency at which the speed of an incident acoustical wave in air matches that of a glass bending wave. At the coincident frequency, the acoustic wave is especially effective at causing glass to vibrate, and the vibrating glass is an effective sound radiator at or near the coincident frequency and at frequencies above or below the coincident frequency. As a result, glass exhibits a dip or decrease in sound transmission loss, referred to as the coincidence dip or coincident effect, and the glass becomes transparent to sound.
The coincident frequency can be represented by the following equation (1):fc=c2/2π×[ρs/B]1/2  (1)
where c is the sound speed in air, ρs is the surface density of the glass panel, and B is the bending stiffness of glass panel. In general, the coincident frequency increases with decreasing thickness of the glass panel. For automotive glazings, the coincident frequency is typically in the range of 3150 to 6000 Hz, which is well within the wind noise frequency region of 2000 to 8000 Hz. For laminated architectural building glass (such as windows), the coincident frequency is generally less than about 3150 Hz.
The coincident effect not only results in a dip or decrease in sound transmission loss at the coincident frequency, but also reduces sound transmission loss at frequencies above and below the coincident frequency. Glass panels exhibiting severe coincident effect (low STL) at the coincident frequency will transmit sound more dominantly at that frequency, resulting in an enclosed area (such as the interior of a car or a room in a building) with high sound intensity at or near the coincident frequency. It is therefore important to characterize the sound insulation properties of a glass panel by weight averaging its sound transmission loss in the frequency of interest, such as in the coincident frequency region, and the sound transmission loss at the coincident frequency (as further described herein).
The thermoplastic polymer found in safety glass can consist of a single layer of a thermoplastic polymer, such as poly(vinyl acetal) or poly(vinyl butyral) (PVB), that has had one or more physical characteristics modified in order to increase acoustic dampening and reduce the sound transmission through the glass. Conventional attempts at such acoustic dampening have included using PVB interlayers with low glass transition temperatures. Other attempts have included multilayer interlayers having two adjacent layers of thermoplastic polymer wherein the layers have dissimilar characteristics (see, for example U.S. Pat. Nos. 5,340,654, 5,190,826, and 7,510,771). These multilayer interlayers typically comprise a soft inner or core layer and two stiffer outer or skin layers. The soft core layer provides acoustic damping properties, while the stiff skin layers provide handling, processability, and mechanical strength of the interlayer.
Despite these advances, there is a continued need to further improve the acoustic properties and reduce the sound or noise transmission through the coincident frequency region, especially in the wind noise frequency region. Accordingly, there is a need in the art for the development of an interlayer, either a monolithic or a multilayer interlayer, that provides improved acoustic or sound insulation properties without a reduction in other optical, mechanical, and physical characteristics of an interlayer, and a need for the development of polymer interlayers including monolithic and multilayer interlayers that provide improved acoustic properties for glass panels when used in vehicle and building glazings.